1. Field of the Invention
The present invention relates to an oxide thin film transistor (TFT) and a fabrication method thereof, and more particularly, to an oxide TFT using an oxide semiconductor as an active layer, and a fabrication method thereof.
2. Description of the Related Art
As the consumers' interest in information displays is growing and the demand for portable (mobile) information devices is increasing, research and commercialization of light and thin flat panel displays (“FPD”), which substitute cathode ray tubes (CRTs), the conventional display devices, has increased. Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images by using optical anisotropy of liquid crystal. LCD devices exhibit excellent resolution, color display and picture quality, so they are commonly used for notebook computers or desktop monitors, and the like.
The LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal molecules in a pixel unit are driven by using amorphous silicon thin film transistors (a-Si TFTs) as switching elements.
The structure of a related art LCD will now be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing a related art LCD device.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes a plurality of gate lines 16 and a plurality of data lines 17 which are arranged vertically and horizontally to define a plurality of pixel areas (P), TFTs (T), switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel areas (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrate 5 and the array substrate 10 is made by an attachment key (not shown) formed on the color filter substrate 5 or the array substrate 10.
The LCD as described above is light and has low power consumption, and as such, the LCD receives much attention, but the LCD is a light receiving device, not a light emission device, having a technical limitation in brightness, a contrast ratio, a viewing angle, and the like. Thus, the development of a new display device that is able to overcome such shortcomings has been actively made.
An organic light emitting diode (OLED), one of new flat panel display devices, is self-emissive, having an excellent viewing angle and contrast ratio compared to the LCD, and because it does not require a backlight, it can be formed to be lighter and thinner and is advantageous in terms of power consumption. Besides, the OLED can be driven with a low DC voltage and has a fast response speed, and in particular, the OLED is advantageous in terms of fabrication costs.
Recently, research for an increase in the size of an OLED display device has been actively ongoing, and in order to achieve such a large-scale OLED display device, the development of a transistor that can secure constant current characteristics as a driving transistor of an OLED to ensure a stable operation and durability is required.
An amorphous silicon thin film transistor (TFT) used in the above-described LCD may be fabricated in a low temperature process, but it has very small mobility and fails to satisfy a constant current bias condition. Meanwhile, a polycrystalline silicon TFT has high mobility and satisfies constant current bias condition but fails to secure uniform characteristics, making it difficult to increase the area and requiring a high temperature process.
Thus, an oxide semiconductor TFT in which an active layer is formed with oxide semiconductor has been developed. The oxide semiconductor form a large spherical s-orbital based on a material having semiconductor characteristics included in the metal oxide formed as metal and oxygen are bonded, so although it is amorphous, electrons can move easily, implementing fast mobility.
Here, when the oxide semiconductor is applied to an existing TFT having a bottom gate structure, the oxide semiconductor is damaged during a process of etching source and drain electrodes, in particular, during dry etching using plasma.
In order to prevent the problem, an etch stopper is selected to be additionally formed on an upper portion of the active layer, but in this case, it is impossible to uniformly pattern the etch stopper with respect to the entire pixel unit due to a process error, so it is difficult to implement a short channel of 10 μm or less, and a photolithography process (referred to as a ‘photo process’, hereinafter) is disadvantageously added.
FIG. 2 is a sectional view sequentially showing a related art oxide TFT.
As shown in FIG. 2, a related art oxide TFT includes a gate electrode 21 formed on a certain substrate 10, a gate insulating layer 15a formed on the gate electrode 21, an active layer 24 formed of an oxide semiconductor and an etch stopper 25 made of a certain insulating material on the gate insulating layer 15a, source and drain electrodes 22 and 23 electrically connected to certain regions of the active layer 24, a protective film 15b formed on the source and drain electrodes 22 and 23, and a pixel electrode 18 electrically connected to the drain electrode 23.
FIGS. 3A to 3F are sectional views sequentially showing a process of fabricating the related art TFT illustrated in FIG. 2.
As shown in FIG. 3A, a first conductive film is deposited on the entire surface of the certain substrate 10 and then selectively patterned through a photo process to form the gate electrode 21 formed of the first conductive film.
Next, as shown in FIG. 3B, the gate insulating layer 15a and an oxide semiconductor layer made of a certain oxide semiconductor are sequentially deposited on the entire surface of the substrate 10 and selectively patterned by using a photo process to form the active layer 24 made of the oxide semiconductor above the gate electrode 21.
And then, as shown In FIG. 3C, an insulating layer made of a certain insulating material is deposited on the entire surface of the substrate 10, and then, selectively patterned by using a photo process to form the etch stopper 25 made of the insulating material on the active layer 24.
Thereafter, as shown in FIG. 3D, a second conductive film is formed on the entire surface of the substrate 10 with the etch stopper 25 formed thereon, and then, selectively patterned through a photo process to form the source and drain electrodes 22 and 23 formed of the second conductive film and electrically connected to the source and drain regions of the active layer 24 on the active layer 24 and the etch stopper 25.
And then, as shown in FIG. 3E, the protective film 15b is formed on the entire surface of the substrate 10 with the source and drain electrodes 22 and 23 formed thereon, and then, selectively patterned through a photo process to form a contact hole 40 exposing a portion of the drain electrode 23.
And then, as shown in FIG. 3F, a third conductive film is formed on the entire surface of the substrate 10, and then, selectively patterned through a photo process to form the pixel electrode 18 electrically connected to the drain electrode 23 through the contact hole.
In order to fabricate the oxide TFT having the foregoing structure, the additional photo process is required for form the etch stopper, and in addition, it is difficult to implement a short channel of 10 μm or less due to the use of the etch stopper. Namely, implementation of a short channel is required in order to apply the advantage of fast mobility to an organic electro-luminescence device or implement high transmissivity of a high resolution product, but a channel length is determined by a line width of the etch stopper, and since precision is required for a design margin between the gate electrode, the etch stopper, and the source and drain electrodes, it is impossible to uniformly patterning the etch stopper with respect to the entire pixel unit due to a process error, thus making it difficult to implement a short channel of 10 μm or less.